function fn() {
  let p = new Promise((resolve) => {
    setTimeout(() => {
      resolve()
    }, 1000)
  })
  return p
}
let a,b,c
fn().then(res=>{
  a = 'hello'
  return fn()
}).then(res=>{
  b = `lagou`
  return fn()
}).then(res=>{
  c = `I LOVE U`
  console.log(a+b+c)
})